Image sensor systems including at least two image sensors, which record essentially the same scene (so-called stereo camera systems), will be used in the future for object recognition, e.g. within the framework of a driver assistance system. Since the recorded images are evaluated in subsequent systems, the image sensor system has to provide for sure functioning and for ensuring the quality of the images, test its own performance reliability, react when faults appear, and especially inform subsequent systems evaluating the images on the manner of appearing, and possibly the type of the fault. One fault condition, which may impair the performance reliability of the image system or the quality of the images delivered, is the obstruction of the visual range of one of the image sensors by interfering objects. Such cases occur particularly in the field of motor vehicles when, for example, while it is raining, the windshield wiper runs briefly through the camera image or the windshield in front of a camera is dirty. Such an obstruction, for instance, has disturbing effects on a stereoscopic determination of distances to (from) objects.